1. Field of the Invention
The present invention relates to a bearing made of a graphite-dispersed copper-based sintered alloy, the bearing exhibiting superior wear resistance, particularly when used in a motorized fuel pump which is compact and which is to be operated at high speed.
2. Description of Related Art
Bearings which are structural elements of fuel pumps for gasoline engines and which are provided along a flow path of gasoline could be composed of a copper-based cast alloy comprising 10 to 25% of Zn and 10 to 25% of Ni (xe2x80x9c%xe2x80x9d herein and hereinafter indicates xe2x80x9c% by weightxe2x80x9d), the balance being Cu and unavoidable impurities.
As reduction in the weight and enhancement in performance of gasoline engines for automobiles, for example, have been remarkable recently, reduction in the size of fuel pumps has also been desired. However, a higher operation speed, i.e., a higher rotation speed, is required in order to make the conventional fuel pump compact, maintaining its discharge performance, and as a result, gasoline which is taken into the fuel pump flows at a high speed under a high pressure through the flow path formed of a narrower space. Under such conditions, the bearings, which are structural elements of the fuel pump, made of a copper-based cast alloy, wear very quickly, and the life of the bearings is relatively short.
In view of the above situation, the inventors conducted research in order to develop bearings which are suitable for use in fuel pumps which are compact and are to be operated at high speed. As a result, the inventors found that a bearing comprising a graphite-dispersed copper-based sintered alloy which contains 10 to 25% of Zn, 10 to 25% of Ni, 0.1 to 0.9% of P, and 1 to 8% of C, the balance being Cu and unavoidable impurities, and which has a porosity of 5 to 25% exhibits superior wear resistance under conditions in which the bearing is exposed to a flow of a fuel (such as gasoline) at a high speed under a high pressure, and the inventors found that a motorized fuel pump using this bearing comprising the graphite-dispersed copper-based sintered alloy has a longer life even when the fuel pump is used for pumping a fuel containing sulfur or compounds thereof as an impurity. This is because frictional drag on the bearing from the high-pressure high-speed flow of the fuel is moderated by the pores on the bearing, and because even though the greater the number of pores formed, the greater the wear resistance of the bearing is reduced, such a reduction of the wear resistance is off set by a Cuxe2x80x94P compound, which is rigid and is dispersed in the matrix made of a solid solution phase of a Cuxe2x80x94Nixe2x80x94Zn alloy, and by graphite, which is highly lubricating and is also dispersed in the matrix, and in addition, the Cuxe2x80x94Nixe2x80x94Zn alloy, which forms the matrix, has superior strength and resistance to corrosion.
The present invention is based on the above findings in the research, and provides a bearing for a motorized fuel PUMP, which exhibits superior wear resistance to a high-pressure high-speed flow of a fuel (such as gasoline), the bearing comprising a graphite-dispersed copper-based sintered alloy which contains 10 to 25% of Zn, 10 to 25% of Ni, 0.1 to 0.9% of P, and 1 to 8% of C, the balance being Cu and unavoidable impurities, and which has a porosity of 5 to 25%, and provides a motorized pump comprising this bearing.
Next, reasons for limiting the composition and the porosity of the graphite-dispersed copper-based sintered alloy in the bearing of the present invention to the above ranges will be explained in the following.
(1) Composition
(1-a) Zinc (Zn) and Nickel (Ni)
As explained above, Zn and Ni components can be dissolved in Cu in the solid state to form a matrix made of a solid solution phase of a Cuxe2x80x94Nixe2x80x94Zn alloy, and have a function of maintaining the strength and the corrosion resistance of the bearing. With regard to Ni, the strength of the alloy tends to be reduced if the amount of Ni is less than 10% or is more than 25%. With regard to Zn, the corrosion resistance of the alloy tends to be reduced if the amount of Zn is less than 10%, while the strength of the alloy tends to be suddenly reduced if the amount of Zn is more than 25%. Accordingly, the range of the amount of Zn is determined to be 10 to 25%, and the range of the amount of Ni is determined to be 10 to 25%. Preferably, the amount of Zn is 15 to 20%, and the amount of Ni is 15 to 20%.
(1-b) Phosphorus (P)
The P component has a function of contributing to the enhancement of the strength of the bearing by improving the sintered state of the alloy, and has a function of enhancing the wear resistance of the bearing by forming a rigid Cuxe2x80x94P alloy which is dispersed in the matrix. However, if the amount of P is less than 0.1%, desired enhancing effects in the above functions cannot be obtained. On the other hand, if the amount of P is more than 0.9%, the strength of the alloy tends to be suddenly reduced. Accordingly, the range of the amount of P is determined to be 0.1 to 0.9%. Preferably, the amount of P is 0.3 to 0.6%.
(1-c) Carbon (C)
The C component is present mainly as graphite dispersed in the matrix. C component imparts excellent lubrication to the bearing, and thus has a function of contributing to enhancement in the wear resistance of the bearing. However, if the amount of C is less than 1%, desired enhancing effects in the above function cannot be obtained. On the other hand, if the amount of C is more than 8%, the strength of the alloy tends to be considerably reduced. Accordingly, the range of the amount of C is determined to be 1 to 8%. Preferably, the amount of C is 2 to 6%.
(2) Porosity
The pores dispersed in the matrix made of the Cuxe2x80x94Nixe2x80x94Zn alloy have a function of moderating strong frictional drag and surface pressure to which the bearing is subjected by a high-pressure high-speed flow of a fuel, thus inhibiting wear of the bearing by such a flow. However, if the porosity of the alloy is less than 5%, the number of pores dispersed in the matrix is too low to satisfactorily yield the above effect. On the other hand, if the porosity of the alloy is greater than 25%, the strength of the bearing tends to be considerably reduced. Accordingly, the range of the porosity is determined to be 5 to 25%. Preferably, the porosity is 10 to 20%.
The bearing comprising the graphite-dispersed copper-based sintered alloy according to the present invention exhibits superior wear resistance not only as a bearing for a common fuel pump, but also as a bearing for a fuel pump which is compact and which is operated at a high speed, e.g., a bearing which is to be used exposed to a high-speed flow of a fuel (such as gasoline) and is subjected to high surface pressures. Accordingly, the bearing according to the present invention can sufficiently contribute to reduction of weights and enhancement of performances of engines such as gasoline engines.